How do I add an event to Apple’s Calendar using drafts?
Take a look at the example in the Drafts docs here.
As @sylumer mentions, it can be done with scripting to allow a great amount of control, but if you are looking for the most straight forward way, start with the Event action - which ships in the default “Basic” action group or can be installed from the link above.
There are a number of other ready to use actions to work with calendars as well, if you search the Action Directory for “event”, “calendar” and similar terms.
Many also use Drafts along with Fantastical to get natural language parsing features. Details in our Fantastical integration guide.